1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system having a self shield for shielding a disturbance electromagnetic wave radiated inside an examination space of a static homogeneous magnetic field being generated where an object (patient) under examination is placed.
2. Description of the Related Art
In a magnetic resonance imaging system, magnetic resonance signals generated by the object patient excited with a magnetic field and a radio frequency pulse are collected from an entire plane simultaneously, and an MRI image is reconstructed based on the signals. The magnetic resonance signal is very weak. For this reason, it is necessary to prevent the magnetic resonance signal from being mixed with a disturbance electromagnetic wave.
For this purpose, the magnetic resonance imaging system is installed in a shield room for shielding the disturbance electromagnetic wave. Thus, the magnetic resonance signal can be shielded from the disturbance electromagnetic wave.
However, the imaging system includes a patient couch on which a patient is positioned. Therefore, a shield room is required to be a relatively large space. In order to obtain a satisfactory shield effect, the construction cost of the shield room is extremely high.
Japanese Pat. Disclosure (Kokai) No. 59-77348 discloses the following RF self shield. The self shield comprises a conductive member arranged in a housing of the imaging system, and a conductive housing for covering a portion of a patient extending outside the conductive member in a tunnel-like shape.
However, the examination space in the housing is cylindrical, and the interior of the conductive housing has an elongated space. For this reason, a patient in the examination space to some extent cannot receive external light, and external air cannot be supplied therein. Thus, the patient suffers from a feeling of oppression from the conductive housing. In particular, when a patient is very sick, the feeling of oppression adversely affects him in respect to the patient's mental health. In addition, the conductive housing must have an opening with a lid, through which a patient can enter the conductive housing. This results in a need for a very large conductive housing.
Furthermore, the conventional self shield has the following drawbacks.
The X gradient coils, Y gradient coils and Z gradient coils (3-1, 3-2, 3-3 in FIG. 1C) are actually concentrically superimposed within static field magnetic coil 2 and are preferably constructed so as to receive at least a portion of the object therein. The RF transmit/receive coil 4 (FIGS. 1A, and 1C) is also of conventional design and is designed so as to cause the RF magnetic field to be perpendicular to the static magnetic field. The conductive member is arranged near the gradient coil. An eddy current is sometimes induced in the conductive member due to the magnetic gradient pulses from the coil. A magnetic field is generated by the eddy current in the conductive member. The magnetic field is harmful to the receiving characteristics of a receiving coil for receiving the magnetic resonance signal. As a result, an the MRI image may be degraded.
Moreover, the frequency of a current flowing through the conductive member may be tuned to that of the magnetic resonance signal. As a result, magnetic coupling occurs between the receiving coil and the conductive member. An electric wave is thus coupled from the conductive member to the receiving coil. For this reason, the magnetic resonance signal cannot be accurately received, and the MRI image is degraded.